This invention relates to a system for correcting distortion in a power amplifier, more particularly in a power amplifier used in digital mobile communications equipment to amplify a signal modulated by quadrature phase-shift keying (QPSK).
Band-limited QPSK modulation has been employed in recent digital mobile communications systems to utilize the available frequency spectrum more efficiently. In such QPSK systems the output of the modulator contains both amplitude and phase information. Because of its high-power output, the last-stage power amplifier in the transmitter has a non-linear amplitude characteristic, and the phase of the amplified output moreover shifts by an amount that varies with the amplitude. The QPSK signal that emerges from the power amplifier is accordingly distorted in both phase and amplitude. The phase distortion is particularly large in conventional power amplifiers employing bipolar transistors. Various systems for correcting the phase and amplitude distortion by means of negative feedback loops have been proposed.
One such system, described in the 1989-nen Denshi Joho Tsushin Gakkai Shuki Zenkoku Taikai B540 (Papers from the 1989 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers of Japan, B540), prevents amplitude distortion by detecting the difference between the amplitudes of the amplified and unamplified QPSK signals and controlling the amplitude gain of the power amplifier accordingly. Phase distortion is corrected by detecting the phase difference between the amplified and unamplified QPSK signals and controlling a voltage-controlled oscillator, the output of which is fed to the power amplifier to be amplified. This system is extremely expensive because it requires three oscillators: one to generate the original carrier signal, another to convert the frequency of the amplified QPSK signal to the frequency of the QPSK signal produced by the modulator so that their phases can be compared, and the voltage-controlled oscillator mentioned above.
Another system, described in 1989-nen Denshi Joho Tsushin Gakkai Shuki Zenkoku Taikai B541 (Papers from the 1989 Autumn Meeting of the Institute of Electronics, Information, and Communication Engineers of Japan, B541), demodulates the amplified QPSK signal and adjusts the modulating inputs to the modulator according to the demodulated signal. In this case the feedback loop delay characteristic must be flat throughout the transmitting frequency band. This leads to problems of filtering: if a passband filter is provided in the feedback loop, as is desirable to ensure separation of transmit and receive frequencies, the complex delay characteristics of the filter must be compensated for. If the filter is located on the output side of the amplifier, however, it must have extremely strong stopband attenuation characteristics. In either case expensive components are required, and the size and complexity of the feedback loop makes it difficult to test the modulator, power amplifier, and other parts of the loop individually.